Bioreactors culturing mammalian cells are used to make recombinant protein drugs such as growth factors, thrombolytic agents, interferons, interleukins, erythropoietins, colony stimulating factors and various other cytokines, and antibodies. However, in bioreactors, the osmolality of the culture increases as a result of base addition to control pH as well as feedings of nutrients and supplements to supply necessary energy for the culture. Typically, throughout a fed-batch bioreactor run, osmolality increases from about 300 milliosmoles/kg (mOsm) to values sometimes as high as 600 mOsm. It has been shown that compared to 300 mOsm cell cultures, within a range of osmolality (above 340 mOsm and below a threshold comprised between 400 and 450 mOsm) specific productivity of mammalian cells increases while cells' growth rate decreases and cells' death rate is not impacted. For most cell lines there seems to exist an osmolality threshold above which cell death rate increases dramatically with osmolality (often times that thresholds seems to be comprised between 400 and 450 mOsm).
In bioreactors, increased osmolality is correlated with increased tonicity (increased NaCl). High NaCl activates the transcription factor tonicity-responsive enhancer-/osmotic response element-binding protein (TonEBP/OREBP, where TonE is also called “tonicity enhancer,” or “osmotic response element”), which activates TonE, resulting in increased transcription of several protective genes whose promoters are controlled by its cognate binding sites: the enhancer ORE/TonE.
Regulation of TonEBP/OREBP transcriptional activity is complex. Within 30 min of hypertonicity, TonEBP/OREBP becomes phosphorylated and translocates into the nucleus. Hours later, TonEBP/OREBP mRNA and protein abundance increase. Also, hypertonicity increases transactivating activity of TonEBP/OREBP, associated with phosphorylation of its transactivating domain. More slowly, hypertonicity increases TonEBP/OREBP abundance through induction of its mRNA and protein synthesis.